2.1. ETIOLOGY OF AIDS
In 1985 nearly 8,000 people were diagnosed as having AIDS, and the number has been steadily increasing. Fifteen thousand more cases are expected to be diagnosed in 1986, and the number of cases may well double again in 1987. New York Times Magazine, Mar. 2, 1986, p. 42. AIDS has been characterized by the onset of severe opportunistic infections secondary to an effect on the body's immune system. Gottlieb et al., New Eng. J. Med. 305:1426 (1981). The disease has been found in male homosexuals, patients receiving blood products, intravenous drug addicts and individuals originating from Haiti and Central Africa. Piot et al., Lancet 11:65 (1984).
The causative agent was suspected to be of viral origin because the epidemiological pattern of AIDS was consistent with that of a transmissible disease. At least three retroviruses have been isolated from cultured T-cells of several patients with AIDS, or from white blood cells of persons at risk for the disease. A novel human retrovirus called lymphadenopathy-associated virus (LAV) was discovered, and its properties were consistent with an etiological role in AIDS. That virus was isolated from a patient with lymphadenopathy and hence the name. Montagnier et al., in Human T-Cell Leukemia/Lymphoma Virus, R. C. Gallo et al. eds., Cold Spring Harbor Laboratory, pp. 363-370 (1984).
Other human retroviruses, specifically two subgroups of the human T-cell leukemia/lymphoma/lymphotropic virus, types I [Poicsz et al., Proc. Natl. Acad. Sci. U.S.A. 77:7415 (1980)] and III [Popovic et al., Science 224:797 (1984)] have also been isolated. Still another virus, called the AIDS-associated retrovirus (ARV), has been proposed as the causative agent [Levy et al., Science 225:840 (1984)]. Both the HTLV-III and ARV retroviruses display biological and seroepidemiological properties similar to LAV. Levy et al., supra, Popovic et al., supra. Thus, at least three retroviruses have been postulated to be the etiologic agent or AIDS: LAV, ARV, and HTLV-III. For this application, these viruses will be collectively referred to as the AIDS viruses. Because HTLV-III is the prototypic virus for this group, it will be understood that the term "antigenic determinant corresponding to the sequences of a protein of an HTLV-III virus" actually refers to the sequences of the proteins of any of the AIDS viruses.
One reason for the difficulty in determining the true etiologic agent of AIDS was cross-reactivity of various retroviral antigens with serum samples from AIDS patients. For example, serum samples from AIDS patients have been shown to react with antigens of both HTLV-I [Essex et al., Science 220:859 (1983)] and HTLV-III [Sarngadharan et al., Science 224:506 (1984)]. Envelope gene products of HTLV demonstrated antigenicities that were cross-reactive with antibodies in sera from adult T-cell leukemia patients. Kiyokana et al., Proc. Natl. Acad. Sci. U.S.A. 81:6202 (1984). Adult T-cell leukemias (ATL) differ from acquired immune deficiency syndrome (AIDS) in that HTLV-I causes T-cell malignancies that are characterized by the uncontrolled growth of T-cells. In AIDS, instead of cell growth there is cell death. In fact this cytopathic characteristic of HTLV-III was critical to ultimately determining the specific retroviral origin of the disease.
The etiologic agent of AIDS was isolated by the use of immortalized human neoplastic T cell lines (HT) infected with the cytopathic retrovirus characteristic of AIDS, isolated from AIDS afflicted patients. Seroepidemiological assays using this virus showed a complete correlation between AIDS and the presence of antibodies to HTLV-III viral antigens. Gallo et al., supra, 1984; Sarngadharan et al., supra, 1984; Schupbach et al., Science 224:503 (1984). In addition, nearly 85% of patients with lymphadenopathy syndrome and a significant proportion of asymptomatic homosexual men in AIDS endemic areas were also found to carry circulating antibodies to HTLV-III. Taken together, these data implicate HTLV-III as the principal etiologic agent for AIDS.
Until the successful culturing of the AIDS virus using the H-9 cell line, the env AIDS protein of the AIDS virus had not been isolated, characterized or synthesized. This in major part was due to the fact that the virus is cytopathic, and isolation of the virus was thus not possible. Popovic et al., Science 224:497 (1984). Once a human T-cell line that was resistant to the cytopathic effects of the virus was discovered, however, the molecular cloning of AIDS proviral DNA could be carried out.
The need for sensitive and rapid methods for the diagnosis of AIDS in human blood and in other biological fluids and for a method to prevent the disease by vaccination is very great. Virtually all of the assays and tests presently available are fraught with errors. In fact, the Center for Disease Control (CDC) has indicated that presently available tests should be used solely for screening units of blood for the presence of antibodies to HTLV-III. The CDC has even gone further by stating that the presently available enzyme-linked immunosorbent assay (ELISA) tests should not be used for the general screening of high risk populations or as a diagnostic test for AIDS. Federal Register 50(48):9909, Mar. 12, 1985.
The errors in previously used AIDS tests have been traced to the failure to use a specific antigenic protein of the etiologic agent for AIDS. The previously used proteins were derived from a viral lysate. Since the lysate is made from human cells infected with the virus, i.e. the cells used to grow the virus, the lysate will contain human proteins as well as viral proteins. Thus preparation of a pure antigen of viral protein is very difficult. The antigens used until now have thus produced both false positive and false negative results. Budiansky, Nature 312:583 (1984). The errors caused by the use of such lysate proteins/peptides could be avoided by using a composition for binding AIDS antibodies which is substantially free of the non-AIDS specific proteins. Compositions of substantially pure AIDS envelope and core protein can be used as antigens.
Both the envelope and core proteins of HTLV-III have conserved antigenic determinants which would permit their use to screen for, diagnose and/or provide protection by vaccination against the AIDS viruses. And individuals that have been exposed to HTLV-III and who may thus be at risk to contract AIDS or who have the disease can be identified by the presence in their blood of antibodies to the viral core protein (gag) and/or the envelope protein (env). Sarngadharan et al., Science 224:506 (1984).
The availability of a reliable and sensitive test for the presence in blood or in other biological fluids of the AIDS virus itself, or of particles therefrom, is also important. Groopman et al. [Blood 66:742 (1985)] have reported that antibodies against AIDS viruses are not always present in the blood of AIDS victims. Groopman et al. examined one patient with AIDS and another with related disorders (ARC) from whom blood samples were taken which were antibody negative but from which HTLV-III could be cultured.